In manufacturing processes involving generally flat plate-like workpieces, it is a standard practice to secure the workpiece in a defined location on a flat work surface such as a vacuum chuck by registering two orthogonal edges of the workpiece against three registration points defining a corner. This is the common practice, for example, in the hybrid circuits manufacturing process, wherein ceramic plates are used as substrates upon which circuitry is created by the silkscreening of conductive and resistive pastes. Subsequently, resistors within the circuitry are modified, usually by automatic laser trimming, to adjust their resistance to within a specified tolerance of the desired value. In addition, active electronic components are often incorporated within the circuit; the loading of these components is also generally done by automatic equipment. Repeatable registration of the workpiece relative to the workholders of the various machines in the production line is essential if expensive and time consuming alignment procedures for each workpiece are to be avoided.
Registration of the workpieces may be done manually or mechanically. Manual registration is extremely simple in concept: the operator places the workpiece on the work surface (e.g., vacuum chuck) and slides it into contact with the three registration points (usually tooling pins); the human hand has no difficulty ascertaining that the workpiece has made contact with all three registration points and that no more force or motion is required. Mechanical registration is necessary when the workpieces are to be loaded by automatic machines (e.g., "pick and place" robots) without operator intervention. When such a machine places a workpiece on the work surface, the workpiece will in general need to be translated in two axes (e.g., X and Y) and rotated through some angle (Theta) in order to make contact with the registration points. Mechanical registration has generally been accomplished by incorporating in the workholder of each process machine some form of gripping mechanism, typically including a pair of spring-loaded arms which push the workpiece into the registration corner from the variable position at which it was deposited by the loader. Such mechanisms work well when configured for a particular size and shape of workpiece, but their reliability can suffer when a variety of workpiece sizes must be accommodated; and they represent a certain level of complexity since they contain in general several moving parts. PG,4